The function and consequences of fluorescence in tetrapods.

Fluorescence, the optical phenomenon whereby short-wavelength light is absorbed and emitted at longer wavelengths, has been widely described in aquatic habitats, in both invertebrates and fish. Recent years have seen a stream of articles reporting fluorescence, ranging from frogs, platypus, to even fully terrestrial organisms such as flying squirrels, often explicitly or implicitly linking the presence of fluorescence with sexual selection and communication. However, many of these studies fail to consider the physiological requirements of evolutionary stable signaling systems, the environmental dependence of perception, or the possible adaptive role of fluorescent coloration in a noncommunicative context. More importantly, the idea that fluorescence may simply constitute an indirect by-product of selection on other traits is often not explored. This is especially true for terrestrial systems where environmental light conditions are often not amenable for fluorescent signaling in contrast to, for example, aquatic habitats in which spectral properties of water promote functional roles for fluorescence. Despite the appeal of previously unknown ways in which coloration may drive evolution, the investigation of a putative role of fluorescence in communication must be tempered by a realistic understanding of its limitations. Here, we not only highlight and discuss the key body of literature but also address the potential pitfalls when reporting fluorescence and how to solve them. In addition, we propose exciting different research avenues to advance the field of tetrapod fluorescence.

Body size and substrate use affect ventral, but not dorsal, brightness evolution in lizards.

Substrate properties can affect the thermal balance of organisms, and the colored integument, alongside other factors, may influence heat transfer via differential absorption and reflection. Dark coloration may lead to higher heat absorption and could be advantageous when substrates are cool (and vice versa for bright coloration), but these effects are rarely investigated. Here, we examined the effect of substrate reflectance, specific heat capacity (cp), and body size on the dorso-ventral brightness using 276 samples from 12 species of cordylid lizards distributed across 26 sites in South Africa. We predicted, and found, that bright ventral colors occur more frequently in low cp (i.e., drier, with little energy needed for temperature change) substrates, especially in larger body-sized individuals, possibly to better modulate heat transfer with the surrounding environment. By contrast, dorsal brightness was not associated with body size nor any substrate thermal property, suggesting selection pressures other than thermoregulation. Ancestral estimation and evolutionary rate analyses suggest that ventral brightness rapidly differentiated within the Cordylinae starting 25 Mya, coinciding with an aridification period, further hinting at a thermoregulatory role for ventral colors. Our study indicates that substrate properties can have a direct role in shaping the evolution of ventral brightness in ectotherms.

Untangling the structural and molecular mechanisms underlying colour and rapid colour change in a lizard, Agama atra.

With functions as diverse as communication, protection and thermoregulation, coloration is one of the most important traits in lizards. The ability to change colour as a function of varying social and environmental conditions is thus an important innovation. While colour change is present in animals ranging from squids, to fish and reptiles, not much is known about the mechanisms behind it. Traditionally, colour change was attributed to migration of pigments, in particular melanin. More recent work has shown that the changes in nanostructural configuration inside iridophores are able to produce a wide palette of colours. However, the genetic mechanisms underlying colour, and colour change in particular, remain unstudied. Here we use a combination of transcriptomic and microscopic data to show that melanin, iridophores and pteridines are the main colour-producing mechanisms in Agama atra, and provide molecular and structural data suggesting that rapid colour change is achieved via melanin dispersal in combination with iridophore organization. This work demonstrates the power of combining genotypic (gene expression) and phenotypic (microscopy) information for addressing physiological questions, providing a basis for future studies of colour change.

Evolution of eggshell structure in relation to nesting ecology in non-avian reptiles.

Amniotic eggs are multifunctional structures that enabled early tetrapods to colonize the land millions of years ago, and are now the reproductive mode of over 70% of all terrestrial amniotes. Eggshell morphology is at the core of animal survival, mediating the interactions between embryos and their environment, and has evolved into a massive diversity of forms and functions in modern reptiles. These functions are critical to embryonic survival and may serve as models for new antimicrobial and/or breathable membranes. However, we still lack critical data on the basic structural and functional properties of eggs, particularly of reptiles. Here, we first characterized egg shape, shell thickness, porosity, and mineralization of eggs from 91 reptile species using optical images, scanning electron microscopy, and micro computed tomography, and collected data on nesting ecology from the literature. We then used comparative analyses to test hypotheses on the selective pressures driving their evolution. We hypothesized that eggshell morphology has evolved to protect shells from physical damage and desiccation, and, in support, found a positive relationship between thickness and precipitation, and a negative relationship between porosity and temperature. Although mineralization varied extensively, it was not correlated with nesting ecology variables. Ancestral state reconstructions show thinning and increased porosity over evolutionary time in squamates, but the opposite in turtles and crocodilians. Egg shape, size, porosity and calcification were correlated, suggesting potential structural or developmental tradeoffs. This study provides new data and insights into the morphology and evolution of reptile eggs, and raises numerous questions for additional research.

Substrate thermal properties influence ventral brightness evolution in ectotherms.

The thermal environment can affect the evolution of morpho-behavioral adaptations of ectotherms. Heat is transferred from substrates to organisms by conduction and reflected radiation. Because brightness influences the degree of heat absorption, substrates could affect the evolution of integumentary optical properties. Here, we show that vipers (Squamata:Viperidae) inhabiting hot, highly radiative and superficially conductive substrates have evolved bright ventra for efficient heat transfer. We analyzed the brightness of 4161 publicly available images from 126 species, and we found that substrate type, alongside latitude and body mass, strongly influences ventral brightness. Substrate type also significantly affects dorsal brightness, but this is associated with different selective forces: activity-pattern and altitude. Ancestral estimation analysis suggests that the ancestral ventral condition was likely moderately bright and, following divergence events, some species convergently increased their brightness. Vipers diversified during the Miocene and the enhancement of ventral brightness may have facilitated the exploitation of arid grounds. We provide evidence that integument brightness can impact the behavioral ecology of ectotherms.

Proteomic and functional variation within black snake venoms (Elapidae: Pseudechis).

Pseudechis (black snakes) is an Australasian elapid snake genus that inhabits much of mainland Australia, with two representatives confined to Papua New Guinea. The present study is the first to analyse the venom of all 9 described Pseudechis species (plus one undescribed species) to investigate the evolution of venom composition and functional activity. Proteomic results demonstrated that the typical Pseudechis venom profile is dominated by phospholipase A2 toxins. Strong cytotoxicity was the dominant function for most species. P. porphyriacus, the most basal member of the genus, also exhibited the most divergent venom composition, being the only species with appreciable amounts of procoagulant toxins. The relatively high presence of factor Xa recovered in P. porphyriacus venom may be related to a predominantly amphibian diet. Results of this study provide important insights to guide future ecological and toxinological investigations.

Beyond colour: consistent variation in near infrared and solar reflectivity in sunbirds (Nectariniidae).

The visible spectrum represents a fraction of the sun's radiation, a large portion of which is within the near infrared (NIR). However, wavelengths outside of the visible spectrum that are reflected by coloured tissues have rarely been considered, despite their potential significance to thermal effects. Here, we report the reflectivity from 300 to 2100 nm of differently coloured feathers. We measured reflectivity across the UV-Vis-NIR spectra of different (a) body parts, (b) colour-producing mechanisms and (c) sexes for 252 individuals of 68 sunbird (family: Nectariniidae) species. Breast plumage was the most reflective and cap plumage the least. Female plumage had greater reflectivity than males. Carotenoid-based colours had the greatest reflectivity, followed by non-iridescent and iridescent melanin-based colours. As ordered arrays of melanin-filled organelles (melanosomes) produce iridescent colours, this suggests that nanostructuring may affect reflectance across the spectrum. Our results indicate that differently coloured feathers consistently vary in their thermal, as well as obvious visual, properties.

Correlation between ontogenetic dietary shifts and venom variation in Australian brown snakes (Pseudonaja).

Venom is a key evolutionary trait, as evidenced by its widespread convergent evolution across the animal kingdom. In an escalating prey-predator arms race, venoms evolve rapidly to guarantee predatory or defensive success. Variation in venom composition is ubiquitous among snakes. Here, we tested variation in venom activity on substrates relevant to blood coagulation among Pseudonaja (brown snake) species, Australian elapids responsible for the majority of medically important human envenomations in Australia. A functional approach was employed to elucidate interspecific variation in venom activity in all nine currently recognised species of Pseudonaja. Fluorometric enzymatic activity assays were performed to test variation in whole venom procoagulant activity among species. Analyses confirmed the previously documented ontogenetic shift from non-coagulopathic venom in juveniles to coagulopathic venom as adults, except for the case of P. modesta, which retains non-coagulopathic venom as an adult. These shifts in venom activity correlate with documented ontogenetic shifts in diet among brown snakes from specialisation on reptilian prey as juveniles (and throughout the life cycle of P. modesta), to a more generalised diet in adults that includes mammals. The results of this study bring to light findings relevant to both clinical and evolutionary toxinology.

Rapid Radiations and the Race to Redundancy: An Investigation of the Evolution of Australian Elapid Snake Venoms.

Australia is the stronghold of the front-fanged venomous snake family Elapidae. The Australasian elapid snake radiation, which includes approximately 100 terrestrial species in Australia, as well as Melanesian species and all the world's sea snakes, is less than 12 million years old. The incredible phenotypic and ecological diversity of the clade is matched by considerable diversity in venom composition. The clade's evolutionary youth and dynamic evolution should make it of particular interest to toxinologists, however, the majority of species, which are small, typically inoffensive, and seldom encountered by non-herpetologists, have been almost completely neglected by researchers. The present study investigates the venom composition of 28 species proteomically, revealing several interesting trends in venom composition, and reports, for the first time in elapid snakes, the existence of an ontogenetic shift in the venom composition and activity of brown snakes (Pseudonaja sp.). Trends in venom composition are compared to the snakes' feeding ecology and the paper concludes with an extended discussion of the selection pressures shaping the evolution of snake venom.